Waste heat from exhaust gases has been used to reduce energy costs in Brayton-cycle and Diesel-cycle combustion processes for a number of years. Such waste heat is recovered by either extracting the heat in a boiler without additional combustion of a fuel, or by using the hot exhaust gas as a preheated oxidant, which is combusted with additional fuel. For new power generation installations using gas turbines, the extraction of heat from the hot exhaust gas, without additional combustion, using a heat recovery steam generation (HRSG) system is preferred due to the increased efficiency of such a combined cycle. However, the capital cost of a heat recovery steam generation (HRSG) system necessary for extracting heat from the exhaust gas is high. Further, for new installations, NOx emissions from the gas turbine typically must be reduced using a flue gas denitrification system. The need for a denitrification system further increases the cost of the combined cycle process.
In retrofitting existing power generation installations, often referred to as "repowering", and to make efficient use of the existing facility while generating additional power, the use of a gas turbine topping cycle is known In a gas turbine topping cycle, the hot exhaust gas from a power-generating gas-fed turbine is used as the above-described hot exhaust gas. An HRSG system can be used with such an installation. However, due to the previously described high cost of the HRSG system, it is often more economical, in retrofitting an existing power generating installation, to recover the sensible heat from the hot turbine exhaust gasses using the existing fully-fired steam generation system, a process commonly referred to as "Hot-Windbox Repowering".
In Hot-Windbox Repowering systems, the hot exhaust gas is used as a preheated oxidant and replaces the combustion air in an existing air-fuel fired boiler. A typical boiler has two sections; a high temperature radiant section and a lower temperature, convection section. The turbine exhaust gas has a lower oxygen content than the normally used combustion air. Because a greater volume of lower oxygen-content hot exhaust gas is needed to burn the same amount of fuel in the radiant section, less energy becomes available in the radiant section. As a result, the flue gas leaving the radiant section contains a higher level of sensible heat. The higher level of sensible heat in the flue gas leaving the radiant section of the boiler, in turn, increases the amount of energy available in the convection section. This causes an imbalance in the operation of the boiler, results in inefficiency and often derates the capacity of the boiler.
A number of boiler modifications are therefore required to allow for the change of oxidant from air to hot turbine exhaust gas, and the resulting higher flue gas temperature and volume. Additional feed water preheater tubes and economizer tubes may need to be placed in, and the air heater may need to be removed from, the convection section New burners and a new windbox may be required to handle the hot exhaust gas and provide adequate flame stability. For coal-fired boilers, the heat input may have to be reduced significantly to avoid erosion of the boiler tubes (i.e., to maintain the gas velocity within the original design limits). In general, each of these modifications significantly increases the cost of the retrofit.
The recovery of sensible heat from an exhaust gas in a conventional combined cycle power generation system (a Rankine-cycle combustion process combined with a turbine topping cycle, has, to date, required a costly HRSG system including a specially designed convective boiler in combination with a steam turbine. Alternatively, in recovering turbine waste heat through an existing boiler with supplemental fuel firing, extensive modifications to the boiler have been required. Such retrofits are often not considered economically attractive due to the aforementioned high cost of retrofitting and the lower overall power generation efficiency of the hot windbox repowering system, compared to the combined cycle provided with a HRSG system.
Another disadvantage of repowering is the increased NOx emission from the gas turbine and boiler. In order to meet stringent NOx emission regulations, a catalytic deNOx system is typically required to reduce the level of NOx in the flue stream, which further increases the cost of repowering.
Accordingly, it is an object of this invention to provide an apparatus and method for recovering sensible heat from a hot exhaust gas that can be used in combination with an existing air-fuel fired combustion process, without requiring extensive modifications to the existing equipment.
A further object of the invention is to provide an apparatus and method, as described above, in which the level of NOx in the gas turbine exhaust gas is reduced in the downstream combustion process so that the overall NOx emission from the converted system does not exceed the NOx emission level of the air-fuel fired system prior to conversion.